Former External Affairs Minister K Natwar Singh on Thursday resigned from the Rajya Sabha as well as Congress with which he was associated for over two decades.

He met Rajya Sabha Chairman Hamid Ansari in the day and later sent his resignation letter.

The septuagenarian leader said he has resigned from the primary membership of Congress and a letter to this effect was sent on to party President Sonia Gandhi [Images].

Singh, once a close associate of Gandhi, turned her critic after the Iraqi oil-for-food scam broke out and he was forced to resign from the government in November 2005.

He had announced his decision to leave Congress at a BJP rally in Jaipur last week in the presence of Rajasthan Chief Minister Vasundhara Raje.

"You will come to know," was his response when asked about his course of action and whether he would be joining BJP.